Add a note about Babel

Author: gaearon

packages/react-scripts/template/README.md

@@ -225,6 +225,8 @@ Note that **the project only includes a few ES6 [polyfills](https://en.wikipedia
 
 If you use any other ES6+ features that need **runtime support** (such as `Array.from()` or `Symbol`), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.
 
+Also note that using some newer syntax features like `for...of` or `[...nonArrayValue]` causes Babel to emit code that depends on ES6 runtime features and might not work without a polyfill. When in doubt, use [Babel REPL](https://babeljs.io/repl/) to see what any specific syntax compiles down to.
+
 ## Syntax Highlighting in the Editor
 
 To configure the syntax highlighting in your favorite text editor, head to the [relevant Babel documentation page](https://babeljs.io/docs/editors) and follow the instructions. Some of the most popular editors are covered.